Types of Shock
Four Major Categories of Shock
Shock is classified into four major categories based on the primary organ system involved: hypovolemic (blood/fluid compartment), distributive (vascular system), cardiogenic (cardiac dysfunction), and obstructive (circulatory blockage). 1, 2
1. Hypovolemic Shock
Pathophysiology
- Results from absolute intravascular volume loss leading to inadequate tissue perfusion 2, 3
- Decreased cardiac output occurs secondary to insufficient preload 4
Hemodynamic Profile
- Decreased cardiac index (<2.2 L/min/m²) 1
- Decreased central venous pressure (CVP) 1
- Decreased pulmonary capillary wedge pressure (PCWP) 1
- Increased systemic vascular resistance (SVR) as compensatory mechanism 1
- Tachycardia and decreased pulse pressure from reduced stroke volume 1
- Decreased mixed venous oxygen saturation (SvO2 <70%) 1
Management
- Immediate aggressive fluid resuscitation with balanced crystalloids is the definitive therapy—vasopressors are NOT primary treatment 5
- Vasopressors may only be used transiently for life-threatening hypotension while simultaneously achieving hemorrhage control and volume restoration 5
- Definitive control of bleeding source is essential 5
2. Distributive Shock
Pathophysiology
- State of relative hypovolemia from pathological redistribution of intravascular volume due to widespread vasodilation 2
- Most commonly septic shock, but includes neurogenic and anaphylactic shock 3, 6
Hemodynamic Profile
- Normal or increased cardiac index in early stages 1
- Decreased systemic vascular resistance (SVR) from pathological vasodilation 1, 7
- Normal or decreased PCWP 1
- Normal or decreased CVP 1
- Clinical presentation includes hypotension, warm extremities, and elevated lactate 1
Neurogenic Shock Specifics
- Presents with bradycardia (not tachycardia) due to loss of sympathetic tone 7
- Decreased SVR without compensatory tachycardia distinguishes it from other shock types 7
- Does NOT show elevated CVP or PCWP seen in cardiogenic shock 7
Management
- Norepinephrine is first-line vasopressor after adequate fluid resuscitation with balanced crystalloids 5
- Target mean arterial pressure (MAP) ≥65 mmHg 5, 7
- Add vasopressin (up to 0.03 units/min) if hypotension persists despite norepinephrine 5
- For neurogenic shock specifically: vasopressors are first-line after spinal immobilization, with norepinephrine preferred 7
- Fluid resuscitation accompanies vasopressor therapy, though primary problem is vasodilation rather than volume depletion 7
3. Cardiogenic Shock
Pathophysiology
- Results from primary cardiac dysfunction with inadequate cardiac output despite adequate preload 1, 2
- Most commonly caused by acute myocardial infarction (7-10% of AMI cases) 1
- Associated with 30-day mortality of 40-45% despite contemporary treatment 1
Hemodynamic Profile
- Decreased cardiac index (<2.2 L/min/m²) 1
- Increased PCWP (>15 mmHg, often >20 mmHg) from left ventricular failure 1
- Increased CVP (>15 mmHg) from elevated right-sided filling pressures 1
- Increased SVR as compensatory mechanism 1
- Tachycardia (attempting to maintain cardiac output), decreased pulse pressure, decreased SvO2 1
- Clinical signs: pulmonary edema, jugular venous distension, cool extremities, altered mental status, oliguria 1
SCAI Classification Stages
- Stage A (at risk): Normal hemodynamics, normotension, clear lungs, normal perfusion 8, 1
- Stage B (beginning shock): Clinical evidence of relative hypotension or tachycardia without hypoperfusion 8, 1
- Stage C (classic shock): Hypoperfusion requiring intervention (inotropes, pressors, or mechanical support) beyond volume resuscitation 8, 1
- Stage D (deteriorating/doom): Similar to Stage C but worsening despite initial interventions 8, 1
- Stage E (extremis): Cardiac arrest with ongoing CPR and/or ECMO, requiring multiple interventions 8, 1
Refractory Cardiogenic Shock Criteria
- Cardiac power output (CPO) <0.6 W is the most critical threshold 1
- Persistent tissue hypoperfusion despite adequate doses of two vasoactive medications and treatment of underlying etiology 1
- Cardiac index <2.2 L/min/m² despite vasopressor and inotropic support 1
- Systolic blood pressure <80 mmHg despite maximal treatment 1
Management
- For AMI-related cardiogenic shock: immediate coronary angiography within 2 hours with intent to revascularize 1
- Norepinephrine is first-line vasopressor when MAP needs pharmacologic support, particularly with tachycardia 1, 5
- Dobutamine is first-line inotrope when signs of low cardiac output persist (up to 20 μg/kg/min) 1, 5
- Consider dopamine only if bradycardia is present 5
- Use phenylephrine or vasopressin in afterload-dependent states (aortic stenosis, mitral stenosis) 5
- Consider temporary mechanical circulatory support when end-organ function cannot be maintained pharmacologically 1
- Intra-aortic balloon pump (IABP) is NOT routinely recommended except for mechanical complications 1
- Target cardiac index >2.0 L/min/m² with PCWP <20 mmHg 1
4. Obstructive Shock
Pathophysiology
- Results from mechanical obstruction to circulation causing hypoperfusion due to elevated resistance 2
- Common causes include massive pulmonary embolism, tension pneumothorax, cardiac tamponade 3, 6
Hemodynamic Profile
- Elevated CVP from impaired venous return or right heart obstruction 1
- Decreased cardiac output from mechanical obstruction 6
- Variable SVR depending on specific etiology 6
Management
- Immediate life-saving intervention to relieve obstruction is definitive treatment 2
- Fluid challenge may optimize preload before definitive intervention 1
- Specific interventions depend on cause: needle decompression for tension pneumothorax, pericardiocentesis for tamponade, thrombolysis/embolectomy for massive PE 3, 6
Critical Differentiation Points
Distinguishing Cardiogenic from Hypovolemic Shock
Both present with decreased cardiac output, tachycardia, decreased pulse pressure, and decreased SvO2, making differentiation challenging 1
The critical distinguishing feature is ventricular filling pressure:
Clinical examination findings:
- Cardiogenic: pulmonary edema, jugular venous distension, signs of organ hypoperfusion 1
- Hypovolemic: flat neck veins, dry mucous membranes, history of volume loss 1
When diagnosis remains unclear: Invasive hemodynamic monitoring with pulmonary artery catheter provides definitive measurements 1
Point-of-Care Ultrasound Findings
- Cardiogenic shock: Decreased LV contractility, dilated ventricles, B-lines indicating pulmonary edema 1
- Hypovolemic shock: Hyperdynamic LV with small chamber size, collapsed IVC 1
- Obstructive shock: Specific findings based on etiology (RV strain in PE, pericardial effusion in tamponade) 1
General Principles Across All Shock Types
Monitoring
- Arterial catheter placement should occur as soon as practical in all patients requiring vasopressors 5
- Monitor lactate clearance as marker of treatment response 1
- Pulmonary artery catheterization provides definitive hemodynamic measurements in refractory or unclear cases 1
Timing
- Early vasopressor use reduces organ failure incidence 5
- Vasopressors may be initiated during fluid resuscitation and weaned as tolerated 5
- Adequate fluid resuscitation must precede or accompany vasopressor therapy in hypovolemic, cardiogenic, and obstructive shock 5
Common Pitfall
Do NOT confuse late-stage septic shock with cardiogenic shock—septic shock can develop myocardial depression, but the primary hemodynamic pattern remains distributive with decreased SVR 1